1. Field of the Invention
This invention relates in general to an ink jet printer, and more particularly, to a phase control circuit for an ink jet printer.
2. Description of the Prior Art
In an ink jet printer of the type to which the present invention is directed, ink droplets are formed from an ink jet stream discharged under pressure from a nozzle and a charge is applied to the ink droplet which conforms with a desired amount of deflection of the ink droplet necessary to direct the ink droplet to a desired location on a recording medium. In order to bestow the charges on the ink droplets, a charging electrode is disposed near the jet nozzle so that the ink stream discharged from the jet nozzle and the charging electrode form an electrostatic capacity. The electrostatic capacity formed by the ink stream and the charging electrode is controlled by a charging signal applied to the charging electrode, so that as the leading end of the ink stream separates away from the main body of the ink stream, while holding the charge applied thereto, the charged ink droplet is formed. Accordingly, the charging signal corresponding to the desired amount of deflection of the ink droplet must be applied to the charging electrode at precisely the time of formation of the ink droplet if the droplet is to receive a charge conforming with the desired amount of deflection.
The ink droplet having separated from the leading end of the ink stream flies toward a catcher and passes between deflecting electrodes midway of its path to the recording medium. The charged ink droplet is deflected in accordance with the quantity of the charge thereon and the magnitude of the field between the deflecting electrodes, and thereby avoids the catcher to collide against a recording medium and to form a recording dot thereon.
In such an ink jet printer, in order to apply a proper charge on each ink droplet, the charging of the electrostatic capacity formed by the ink stream and the charging electrode must be in a stable state (saturated state) at the moment at which the ink droplet separates from the ink stream. To this end, an electromechanical transducer is mounted on the nozzle and subjects the ink stream to a vibration so as to control the generation of the ink droplets to occur at regular intervals in synchronism with the vibration voltage. However, the vibration voltage to be applied to the electromechanical transducer and the separation phase of the ink droplets are not always constant since the separation phase is also subject to other conditions, such as ink pressure, etc. Accordingly, even when the charging signal in the form of pulses synchronized with the vibration voltage is impressed on the charging electrode, the ink droplet does not always receive the proper quantity of charge due to the ink droplet being formed out of phase with the charging signal.
In order to check the phase of ink droplet formation with respect to the phase of the charging signal, certain ink droplets are charged by a phase detecting signal of known magnitude and pulse width. Then whether or not the particular ink droplet is properly charged is discriminated, and the phase relation between the vibration voltage and the phase detecting signal (or recording signal) is matched. In this case, the central phase of the phase detecting signal is set to be equal to that of the recording signal. A sensor is disposed on the path along which the ink droplet properly charged by the phase detecting signal is deflected and flies, the phase relation is discriminated in dependence on whether or not the ink droplet charged by the phase detecting signal is deflected to collide against the sensor.
During the phase matching period, the ink droplet is charged by the phase detecting signal to match the phase relation, while during the recording period, the ink droplet is properly charged by the recording signal. The pulse width of the phase detecting signal is made smaller than that of the recording signal. In this way, deterioration of the phase relation between the phase detecting signal and the ink droplet can be detected at an earlier time than that between the recording signal and the ink droplet, and hence, the phase matching can be effected at an early stage.